Electro-acoustic transducer



Nov. 10, 1959 H. s. KNOWLES ETAL 2,912,523

ELECTRO-ACOUSTIC TRANSDUCER 2 Sheets-Sheet 1 Filed Oct. 26. 1955mmvrons.

Nov. 10, 1959 H. S. KNOWLES ETA!- ELECTRO-ACOUSTIC TRANSDUCER Filed Oct.26. 1955 2 Sheds-Sheet 2 INVENTORS.

J? YW/n United States Patent 2,912,523 ELECTRO-ACOUSTIC TRANSDUCER HughS. Knowles, Glen Ellyn, and Joseph E. Mullen,

Westmont, Ill., assignors to Industrial Research Products Inc., FranklinPark, Ill., a corporation of Delaware Application October 26, 1955,Serial No. 542,805

12 Claims. (Cl. 179- 108) This invention relates to a transducer andparticularly to an electro-acoustic transducer of a size that may beused in hearing aids. For another electro-acoustic transducer seecopending United States patent application Serial No. 436,416, filedJune 14, 1954 by the present applicant Knowles and another.

The transducer which is the subject of this invention is of that typewherein a vibratable armature is disposed between two gaps, one in eachof two flux conductors connected in parallel in a flux circuit. Undercircumstances where the armature is magnetically entered in each gap,there will be no net flux flow through the armature and the transduceris in magnetic balance. By applying a variable current to a coil aroundthe armature, or by mechanically moving the armature, for example by adiaphragm, this balance is upset, and the resulting flow of flux throughthe armature produces an energy in the alternative form to thatintroduced to the transducer, i.e., if the energy introduced is avariable electric current in the coil, the resulting mechanical movementof the armature vibrates the diaphragm; but if the energy introducedcomes from vibrations of the diaphragm, the resulting flow of fluxthrough the armature induces an electric current into the coil.

The principal object of the present invention is to provide anelectro-acoustic transducer which not only is shielded from externalmagnetic fields, but which does not itself generate such fields.Applicants transducer is used principally as a microphone and as areceiver in a hearing aid, and so long as the microphone and thereceiver were physically spaced from each other, as for example, thereceiver in the ear and the microphone on the chest, the generation bythe transducer of magnetic flux fields extending beyond its physicalboundaries was nottoo important. Applicants directed their attentiontoward shielding the armature and coil of the transducer from externalstray fields, and the acoustic transducer described in theabove-identified copending application was developed.

This object of scaling in magnetic fields and shielding from externalfields has become acute because hearing aid designers wish to mount themicrophone and the receiver close to each other in a single case. Thehearing aid people wish to use accepted articles of headwear such aseyeglasses or earrings, in the case of women, to house both microphoneand receiver. This juxtapositioning of the electric working partsresults in serious feedback and oscillation, particularly since theelectrical power in the receiver is between 105 and 108 times theelectrical power in the microphone in modern hearing aids. Since thesensitivity of these acoustic transducers depends ,upon the efficiencyof the translation of energy from mechanical to electrical or electricalto mechanical forms, it is important that the energy within thetransducer be not added to or decreased by outside apparatus.

One of the features of the present invention is the positioning of thearmature, or reed, in the center of what may be described as a cage offlux conductors. Any

2,912,523 Patented Nov. 10, 1959 "ice magnetic fields generated in theair by the armature or coil of the transducer are held within the limitsof the transducer because expanding they encounter flux conductors whichcapture and confine them. On the other hand, these same flux conductorsact as shields for absorbing any stray fields entering from outside.conductors are the pole pieces and the magnets which are suflicientlyextensive so that they form a sort of tubular wall in the axis of whichlies the armature. They establish such perfect flux paths, i.e., sodevoid of air gaps, that there is little likelihood of flux leaving thesysterm so as to constitute stray fields which interfere with otherapparatus. The relative size of the outside surfaces of the magnets andpole pieces to air gaps leading to the armature is of the utmostimportance.

Another object of this invention is to prevent stray fields fromadversely affecting normal flux flow or flux fields in the transducer.When a stray field encounters a flux conductor in a transducer, thestray flux will either add to or subtract from the flux flowing throughthe conductor. One of the features of this invention is the positioningof flux conductors with reference to both the coil and the reed so-thata single stray field entering the transducer will tend to produce equaland opposite flux flows in the flux conductors and will cancel itselfout.

Another object of this invention is to increase the strength of thetransducer for a given volume. A feature of the present invention is thepositioning of two spaced magnets between the long sides of two polepieces disposed in spaced, parallel, registered relationship. As will beseen in the disclosure, the resulting structure is a sort of a tubeclosed at both ends, except for a through, transverse opening in whichthe coil is positioned in certain embodiments. The flux circuits arevery compact and with the armature positioned in the axis of this tube,the flux from the two magnets can be brought to the working gaps veryeffectively.

A second embodiment of the invention is similar to the first, but showsthe use of two shunt bars made of high flux permeable material, thebasic idea disclosed in the above mentioned copending application. Theshunt bars enclose the coil and act as an additional shield fromexternal stray fields. 7

These advantages have been attained in both embodiments withoutsacrificing simplicity of assembly and without impairing the openingextending to air from both sides of the armature adjacent that portionin the fixed gap whereby a pin may be inserted from either side so as toadjust the free end of the armature in the other gap to produce magneticequilibrium between the two gaps even though the mechanical parts arenot in mechanical symmetry. 1

These and such other objects as may hereinafter appear are'attained inthe embodiments of the invention-shown in the accompanying drawings,wherein:

Figure 1 is a perspective view of a preferred form of applicants twinmagnet transducer;

Figure 2 is a longitudinal sectional view taken on the line 2-2 ofFigure 1;

Figure 3 is a plan view from the diaphragm side of the transducer withthe diaphragm removed;

Figure 4 is a sectional view taken on the line 4-4 of Figure 2; g v

Figure 5 is a sectional view taken on the line 5-5 of Figure 2; e

. Figure 6 is a plan top view of the generator portion ducer of Figures1 to 5 in vertical alignment for assembly;

Figure 8 is a perspective view of the transducer shown.

in Figure 1 with shunt bars in position; and,

Figure 9 is a sectionalview taken on the line 99 ofFigure 8.

Continuing to refer to the drawings, particularly Figures 1 to 5,applicants? transducer consists of two permanentmagn'ets 10 and 12.While these magnets are'e1ongated; the lines of'flux do'not run parallelto thelength but'transverselyas the N and S symbols indicate. Themagnetic strengths of these two magnets are substantially' equal. Thisis attained by selecting for each trans-. ducer similar magnets,assembling them into the transducer; and thendemagnetizing or furthermagnetizing until the'total magnetism reaches an optimum value. Fluxconnecting the two north poles of the magnets 10 and 12 is an O-shapedpole piece 14 andfiux connecting the two southpoles is anO-sh'aped polepiece 16. Each pole piece has twoihwardly directedoifsets 18 and 20, and22 and 24; which will be called gap faces. Between the faces 20and'24are'two spacers 26' and 28 which have permeabilities of the orderof magnitude of that of air between which is 'mounted'one end of aflux-conductive armature or'reed 30. These together constitute the fixedflux gap 31; Between the faces 18 and 22' is mounted a nonmagneticspacer 32 whose thickness preferably equals the combined thickness ofthe spacers 26 and 28 and the reed 30; inwardly of the spacer 32 is thevariable flux gap 33. Itis to be'understood that the distance betweenthe faces 18 and 22 need not'equal the distance between the faces Mind24; nordoes the area of these faces need to be. equal, as longas theends of the reed 30 are disposed in equal magnetic potential'regions inthe two gaps 31 and 33, without excitation;

Positioned around the reed 30 is a coil 35 wound on a fluxnon-conductive'frame 34, which is press fitted into the opening betweenthe O-shaped pole pieces 14 and 16. Referring to Figure 6, the polepiece14 has coil frame seats 23'and 25 which are narrower than the shortestdistance between the faces 18 and 20, thereby providing throughpassageways 50 and 52. Leads 41 and 43 may be brought out-from thecoil'in whatever direction is required by the transducer housing to beused. As can be seen in Figure 2, the opening through the coilsubstantially exceeds the cross-sectional dimensions of the reed 30=sothat the reed '30 is free to vibrate therein. Mounted onthe free end ofthe reed in the passageway 52 is a diaphragm link 36.

The transducer is adjusted according to the application it is to have.If a substantial direct current is to flow through the coil 35' underoperating conditions, as is required of a receiver, the reed30 isadjusted to have its ends in equipotential magnetic regions with thiscurrent flowing through the coil. Likewise, the reed 30 is positioned inequi-magnetic potential regions if no substantial direct current-flowsthrough the coil 35, as when used as. a microphone.

Tlie=alloy composition of'these two pole pieces provides the maximumincremental permeability attainable with the unidirectional flux densityrequired. These pole pieces carry the sustaineduni-directional fieldestablished by the magnets 10"and 12 as indicated by the heavy solidline with the single direction arrows in Figure 1. In this structure byusing two symmetrically disposed magnets, onlyhalf 'of'theuni-directional flux carried by gaps 31 and 33 is carried by each arm ofthe pole pieces. This lowered uni-directional flux permits the selection,of a magnetic alloy having higher permeability. This higherpermeability also improves the shielding elfect of the pole pieces. Aswill be noted from the flux lines in Figure 1, the symmetrical structurealso provides two parallel paths for the alternating flux, therebydoubling the permeance, orthe ease inconducting this alternating flux.Alloys containing 45-50% nickel, the balance iron, provide a' goodcompromise between incremental permeability and saturation 'density inthis application.

It is to be noted that the transducer constitutes a vibration detectorwithoutthe link 36 anddiaphragm62. A

4 a vibrational force applied parallel to the plane of vibration of thereed 30 will set the reed into motion.

In like manner, the transducer described above may be used as arecording head for a phonograph by replacing the link 36 and diaphragm62 with a stylus 37.

These basic elements of applicants transducer are assembled on arectangular mounting plate 44. This plate 44 is made of fluxnon-conductive material and it or equivalent may constitute the solebase plate for the transducer. Where the transducer is to be used as amicrophone or receiver as herein shown, the mounting plate 44 isassembled with a heavier plate 38, also of flux. nonconductive material,whose rectangular perimeter carries an outwardly directed flange 40whichforms an .opensided rectangular sound chamber over/the face ofwhich is positioned a diaphragm 62by any suitable means. The plate 33has a large rectangular opening 42, see Figure 3, in which seats anoffset portion in the plate 44. The plate 44 has an opening 46' throughwhich extends'thelink '36, whose lower end is fastened to the diaphragm62'. The plate 44' also has an opening 48 in alignment withthepassageway 50. A pair of'spacers 53 and S4-arepositioned above the plate44. i

in devices of this type, thousandth s of an inchareim= portant.Referring to Figurel, the offset portion of the plate 44 forms aninwardly directed recess into whichth'e coil 35 extends, therebyreducing. the. gross thicknessof the transducer by of an inch.

All of the parts thus far described are held'in assembled relationshipby two threaded bolts 56 and58, the under side of whose heads engage thediaphragm recep-. tacle side of the mounting plate 38. Their shanks passthrough aligned holes in the various parts as shown in Figures 2 and 4,being drawn tight in threaded holes, such as 59 in the upperpole piece14.

In order that the flux circuits will be confined "to the two magnets,the reed and the pole pieces, .all other parts are made of fluxnon-conductive material, These. parts,

namely, themountingplate 38', plate 44, spacer-s 5'3, 5'4, 26, 28, 32,screws 56 and 58, and link 36, are made of copper, brass or aluminum. 0

In a device as sensitive as applicants transducer, even smallimperfections such as metal shavings that may drop onto the parts duringassembly, or burrs left by the stamping tools, or slight irregularitiesin spacing of the parts, will effect a flow of flux down the reed eventhough the field around the coil is static and the link 36 is. mo;tionless. Applicants make no attempt to correct these errors byattaining .very close mechanicalsymmetry. 'Instead, applicants utilizethe passageways 5'1 andSZ, see Figures 2 .and 3, as a means forinserting a pin from above to deflect the reed downwardly'and from belowto deflect it upwardly so as to magnetically center th'etfree end of thereed 30 in the gap 33Jand attain an'equal'fl'ow of "flux through thegaps 31 and 33 even though the parts are not in exact mechanicalsymmetry.

As illustrated in Figure 2, the transducer is in theform of amicrophone, and has a generally rectangularime perforate case .63 withan open end sealed to the mount: ing plate'38. The case 63, shown bybroken lines, prevents interference from the back wave from thediaphragm 62 with the incident sound waves on the face ofthediaphragm62. The frequency response of the microphone is furtheradjusted to the'range for-hearingaids by the plate 44 with itsaperture'46, and a cover'64,;also shown by broken lines, which issecuredto the mounting plate 38 adjacentto-the diaphragm 62 'and'providedwitl'ian orifice 65 confrontingthe central portion of the diaphragm62. Boththe case 63 and cover 64 are constructed .of non-magnetic materials,suchas aluminum;

In Figures 8 and9, applicants show the-twin-magnet transducer of'Figures1 and 2' equipped, however, with shunt bars and 82. The transducer ofthese figures is identical withthe first form and hence the parts arenot numbered-imorder-that the positioning-of the shunt bars and the fluxcircuit lines may be seen with less confusing indicia around them.Referring to Figure 8, the shunt bar 80 will be seen to be a flat stripof material having two reverse bends such as 84 and 86 at each end.The-shunt bar 80 is identical with the shunt bar 82, but they areinverted with respect to each other so that the coil 32 will be clear.The shunt bars are in physical contact with flux-conductive metal onlyon the faces 88 and 90; otherwise, they are spaced from all other bodiesalthough they may' be in physical contact with the end walls 92 and 94of the coil frame, these end walls being made of flux-nonconductivematerial, i.e., a fibrous material or molded nylon. In order toaccommodate the diaphragm link 36 in the passageway 52. and to maintainthe passageway 50 of a size so that the adjusting pins may be inserted,while nevertheless retaining the same dimensions for the other elementsof the transducer, the width of the coil 32 and its frame have beenslightly reduced.

The two ends of each shunt bar are not symmetrical, the left-hand end asviewed in Figures 7 and 8 being longer so as to permit clamping by meansof the spacer washer 32. Transverse passageways 96 and 98 permit passageof the adjusting pins when they are on the fixed gap side of thetransducer and provide a passageway for the diaphragm link 36 on theother side of the transducer.

In the embodiment, the O-shaped pole pieces 14 and 16 need not be acompromise for handling variable flux and unidirectional flux at thesame time, and hence may be made of rather hard magnetic metal. Theshunt bars 80 and 82 should be made of very soft or high permeabilitymetal. Applicants use Permalloy which has a composition of 79% nickel,4% molybdenum, and 17% iron for shunt bars 80 and 82.

In both embodiments of applicants invention, there is little fluxleakage through air. The sectional view of Figure 5 is substantially thesame for both embodiments of the invention, neglecting the shunt bars inthe embodiment of Figures 8 and 9, and it is evident that the easiestpath for the flux is always through metallic conductors with theexception of the two gaps, and since it' is the purpose of thetransducer to concentrate flux at these two gaps, the maximum strengthattainable from the two permanent magnets is obtained. In the copendingapplication, the permanent magnet circuit has a tendency to pushunidirectional flux into the shunt bars establishing two spacedflux-conductive bodies. Leakage through the air between these twobodies, i.e., shunt bars, occurred and the advantages of the shunt barswere in part lost.

The amount of flux-at the pole face of each of the two gaps 31 and 33 isobtained substantially equally from the two magnets. The surface area ofeach magnet in engagement with the associated pole piece is somewhatgreater than the cross section of both of the arms of the pole pieceleading to the two faces. All of the flux that each magnet can introduceinto the pole piece must flow through these two small cross sections. Inthe copending design, the relationship of this surface area between themagnet and the pole piece and the cross section of the arms leading tothe gaps was closer to 4 or 5 to 1, whereas in the present design, it isonly 2 to 1. By using two smaller magnets and four cross-sectional feedsinto the part of the gap faces, applicants are able to force more fluxto the gaps, or better, are able to relate the sizes of the magnets tothe cross section of the pole piece arms so that substantially all ofthe flux that the magnets can produce will reach the gaps.

Referring to Figures 6 and 7, the opening through one of applicantsO-shaped pole pieces has the general shape of a Greek cross. By thisarrangement, the coil frame can be made with substantially the sameoutside diameter as the inside length of one dimension of the openingand then squeeze-fitted into position. It is important that the coilremain in fixed position with respect to the armature. The side walls ofthe opening of one arm of the cross holds the coil frame. Referring toFigure 5, the coil is entirely inside the outside plane surfaces of theupper and lower pole pieces. This reduces the chance of accidentaldisplacement.

, The ends of the transverse portion of the Greek cross opening providethe narrow passageways 50 and 52 on either side which accommodate thediaphragm link in one passageway and permit insertion of pins in theother passageway to mechanically adjust the armature.

The transducer may be considered as an oblong cage whose top and bottomwalls are formed by the pole pieces and whose side walls are formed bythe magnets with the end of the tube closed by non-magnetic members,such as 32, 26 and 28. Vertically through this cage is an opening inwhich sits the coil. In the long axis of this cage is disposed thearmature or reed. The armature itself is completely shielded from theoutside excepting for comparatively small passageways. A stray fieldapproaching from the outside must first encounter a flux conductor,i.e., pole piece, magnet, or shunt bar. Applicants have measured theelectrical power induced by a stray field in one of these transducersused as a microphone (Fig. 2) and found it to be reduced by a factor offrom 4 to 10 depending on orientation compared to the structuredescribed in the above referenced application, and by a factor of from 9to compared to a device known to the art of generally similarsensitivity and acoustic response. Since the field produced by areceiver is reduced by the same factor for weak signals, and onlyslightly less for strong signals due to non-linear magnetic properties,the power induced by one of'applicants devices used as a receiver intoanother used as a microphone is reduced by an average factor ofapproximately 49 compared to devices described in patent applicationSerial No. 436,416, and by an average factor of approximately 4165compared to prior art devices described above. These figures result fromthe fact that the external field generated by the device is reduced by afactor a and the undesired signal induced into the second similar deviceis reduced by the same factor, thus reducing the electrical couplingbetween the devices by a As a result, the power amplification of ahearing aid can be increased by these respective factors for a givenamount of feedback.

This decrease in electrical coupling between the microphone and receivermakes it possible to juxtapose the microphone and receiver. Further, dueto the small size in which applicants transducers can be constructed,one particular construction being over-all inch by 1 inch by inch, areceiver and microphone may be positioned at distances between centersless than 2 inches and as close as /4 inch between centers, the power,in the receiver exceeding the power in the microphone by a factor of atleast 10 without producing excessive feedback.

In the embodimentshown in Figure 8, the highly permeable shunt bars willhave additional shielding effect, protecting not only the armature butthe coil.

It is also to be noted that the present invention is applicable to alltransducers of the balanced armature type, whether one end of thearmature is clamped or the armature is merely pivoted.

Having thus described their invention, what applicants claim is:

1. A transducer comprising two generally fiat pole pieces, said polepieces being laterally spaced from each other in transverse alignmentand generally parallel relationship, two magnets positioned between thepole pieces and along opposite edges ofthe pole pieces, like poles ofthe magnets contacting the same pole piece, means associated with theinside wall of one pole piece between the magnets and forming two gapfaces so that flux leaves the pole piece through its side wall, saidpole faces formingiwiththe other pole piece two gaps of lowconductivity, an armature positioned in both gaps and vibratable in one,a coil aroundthe armature, a diaphragm, means mechanically connectingthe diaphragm and the armature, and means for holding the foregoingelements in assembled relationship.

2. A transducer comprising a pair of flux-conductive +shaped pole pieceslaterally spaced from and generally parallel to each other, two magnetsdisposed in fluxconductive-relationship. between the pole pieces anddisposed betweenopposite edges of the pole pieces so as to form two fluxgaps between the exposed inner side surfaces ofthe pole pieces, saidmagnets having like poles touching the samev pole piece, aflux-conductive armature having one portion fixed in one gap and anotherportion vibratable in the other gap, and a coil disposedaroundand=spaeed from the armature.

3. A'n-eleotro-acoustic transducer comprising a pair of pole'pieces of'generally flat configuration positioned in spaced relationship withtheir surfaces substantially parallel, two magnets, one positionedbetween the pole pieces along one side and the other positioned betweenthe pole pieces along the other side, the polarity of the magnetstouching the same pole piece being identical, there being centralopening through each pole piece, a coil positioned in said opening, anarmature positioned in said coil and freely movable therein andextending parallel to the sides ofthe pole pieces containing the magnetsand projecting beyond the coil so that the ends of the armature arebetween the inwardly directed'side surfaces of the pole pieces, andmeans for fixing one end of' the armature between the two pole pieces.

4. The transducer of claim 2 wherein the opening in each pole pieceexceeds the length of the coil and the coil is centered in the openingsof the pole pieces thereby providing a passageway between each. end ofthe coil and the adjacentwall of the pole piece.

5. An eleetro-acoustic transducer comprising a pair of flat pole piecespositioned in lateral spaced relationship with their facing surfacessubstantially parallel, two magnets positioned between the pole piecesand spaced from each other with the north pole of both magnets engagingone pole piece and the south poles the other pole piece, there being anopening of Greek cross contigura tion through each pole piece betweenthe magnets so as to establish a passageway through the assembly, anarmature fixedly mounted by one portion between the pole pieces so as toextend between the magnets across the passageway where a free end of thearmature is vibratable between the inwardly directed side surfaces ofthe pole pieces, and a tubular frame carrying a coil press fitted intothe walls of the pole' pieces forming one arm of the cross and disposedaround the armature.

6. A transducer comprising a pair of plate shaped fluxconductive polepieces laterally spaced from and parallel conductive relationshipsbetween the pole pieces and disposed along opposite edges of the polepieces so as to form two flux gaps between the inwardly directedexposedsurfaces of the pole pieces, said magnets having like poles touching thesame pole piece, a flux-conductive armature having one portion fixed inone gap and another portion vibratable inthe other' gap, and a coildisposed around and spaced from the armature, said coil being entirelydisposed between the pole pieces.

7. The transducer of claim 1 wherein the pole faces are inwardly offsetportions of the side surfaces of the pole piece.

8. The transducer of claim 1 wherein the two portions of one of the polepieces whose inner surfaces constitute one side of the two gaps has beeninwardly offset so as to narrow the gap.

9. A transducer comprising two generally flat O-shaped pole pieces, saidpole pieces being laterally spaced fromeach other with the openingstherein in transverse alignment and their surfaces in generally parallelrelationship, twomagnets positioned between the'pole pieces andalongopposite edges of the pole pieces, like poles of the mag nets contactingthe same pole pieces, means associated with the inside wall of those twoportions of one pole piece that connect the magnets forming two gapfaces so that flux leaves the pole piece through its side wall,- saidpole faces forming with the other pole piece two gaps of low fluxconductivity, an armature positioned in both gaps and vibratable in one,a coil around the armature, a diaphragm, a drive pin connecting the dinphragm and the-armature, and means for holding the foregoing elementsinassembled relationship.

10. The transducer of claim 9 wherein the surface.

area of each poleface forming a gap is substantially greater than thecross section of the pole piece carrying flux from one magnet to saidgap.

11. The transducer of claim 9 wherein the means for holding the elementsin assembled relationship "is a pin through both pole pieces and thearmature which clamps the armature in fixed position at one gap.

12. The'transducer of claim 9 wherein each O-shaped pole piece has anoutwardly directed extension through which registering holesarepositioned and which holes carry the pins for holding the elements inassembled relationship.

References Cited in the file of this patent UNITED STATES PATENTS UNITEDSTATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent NO}, 2 9l2 523November 1O 1959 Hugh S. Knowles et a1.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction and that the saidLetters Patent should read as corrected below.

Column 1, line 25 for "entered" read centered Signed and sealed this 9thday of August 1960,

S EAL) Attest:

KARL H AXLINE ROBERT C. WATSON Attesting Oflicer Commissioner of PatentsUNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No".2,912,523 November 10, 1959 Hugh Sa Knowles et a1.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction and that the saidLetters Patent should read as corrected below.

Column 1, line 25,, for "entered" read centered Signed and sealed this9th day of August 1960.

SEAL) Attest:

KARL Hr AXLINE ROBERT C. WATSON Attesting Officer Commissioner ofPatents

